Plant oils in the form of triacylglycerols (TAGs) have twice the energy content of carbohydrates and proteins. Therefore, increasing oil accumulation in vegetative tissues would enhance the energy density and nutritional value of biomass, and could have a major positive impact on the use of biomass as feed and for the production of biofuel and bioelectricity. In addition, using abundant vegetative biomass as vessels of oil accumulation increases the oil storage capacity of plants and has the potential to provide abundant supplies of plant oils for biodiesel production without diverting land from other uses, and thus avoiding a competition between food and biofuel.
Despite the fact that most plant cells have a high capacity to synthesize TAGs, plant vegetative tissues do not accumulate oil to a significant amount. This is because 1) TAGs undergo rapid turnover in plant vegetative tissues; 2) fatty acids, the predominant components of TAG, are used for membrane construction which limits their availability for accumulation in TAGs. Therefore, blocking TAG turnover and increasing fatty acid synthesis represent two key facets of genetic engineering efforts aimed at enhancing TAG accumulation in vegetative tissues of plants.
Previous attempts to enhance fatty acid synthesis mostly focus on transcriptional regulators such as WRINKLED1 (WRI1) that is known to be involved in the activation of fatty acid synthesis genes in oilseeds. However, overexpression of seed-specific transcription factors in vegetative tissues often leads to adverse growth and developmental defects. In addition, overexpression of WRI1 alone does not increase fatty acid synthesis and therefore has had limited positive impact on oil content in plant vegetative tissues.
Prior approaches to blocking TAG turnover include 1) disruption of the peroxisomal transporter1 (PXA1) or sugar-dependent1 TAG lipase responsible for the initiation of TAG breakdown; and 2) Overexpression of seed-specific oil droplet-associated proteins, oleosins, which coat oil droplets (ODs), the TAG storage compartments, thereby blocking oil breakdown by preventing the access of TAG lipases to oil droplets.
We recently reported that overexpression of phospholipid:diacylglycerol acyltransferase1 (PDAT1) boosts both fatty acid and TAG synthesis in leaves. The combined expression of PDAT1 and oleosin1 increased leaf TAG content to 8.6% on a dry weight basis. Several other groups reported that overexpression of WRI1 in combination with diacylglycerol acyltransferase1 (DGAT1) and oleosin results in increased oil production in plant vegetative tissues.
Recently, Kelly et al. (2013) have demonstrated that over expression of WRI and DGAT1 in a sugar-dependent1 lipase-deficient mutant line (sdp1) stimulated TAG accumulation in leaves, stems and roots whereas the sdp1 mutant line accumulated enhanced levels of TAG only in roots and in stems.
According to recent analyses, if 10% of the harvested dry material from high yield biomass crops such as Miscanthus is oil, the oil yield per hectare of such an energy crop would surpass oilseed rape by more than 2-fold. Thus, applying genetic engineering approaches to dedicated bioenergy crops could greatly enhance the nutritional value of vegetative tissues and have the potential to greatly expand the production of plant oils as sustainable sources of biodiesel and industrial chemicals.